libdispatch threads should call GSUnregisterCurrentThread() on exit

[triplef]

Threads started by libdispatch currently don’t clean up their NSThread data when the threads exit. This can cause issues e.g. when [[NSThread currentThread] threadDictionary] is used from such threads as this data will never get released.

libdispatch has a _dispatch_install_thread_detach_callback() hook (currently only available on Android) that can be used for this. We currently use a libdispatch patch to make the hook available on all platforms and install a hook to call GSUnregisterCurrentThread() on thread exit in our app.

It would be nice to install the hook callback from libs-base if the hook is available. To start with I’ll open a libdispatch pull request with our patch.

[triplef]

Opened apple/swift-corelibs-libdispatch#841.

[rfm]

This sounds like some sort of bug ... as far as I can see cleanup should take place irrespective of how a thread was started, libdispatch is in no way a special case and I don't see why it should need an extra callback.
The way it is supposed to work is that GS_THREAD_KEY_INIT registers a a function (via pthread_key_create() or FlsAlloc() on windows) to be called when a thread exits, and that function performs the cleanup. This should work irrespective of what code creates the thread.